[Mart]

It might be, (just a hunch that there was contact between the axle and the floor during extreme bottoming out of the axle. To save things getting bent they may have used something that would give if and when contact was made.

Some roads were a lot rougher then than what we are used to today.

Just a guess, but why else would they do that?

My 33 coupe has a round metal panel in the trunk floor, but a sedan with 3 people in the back could be subject to a lot more weight over the back (especially if a trunk were fitted) and would experience more suspension movement.

Mart.